Infective endocarditis is a disease with significant mortality. The pathogens most commonly associated with this disease are viridans streptococci which may enter the bloodstream during dental procedures. Predisposing factors include rheumatic and congenital heart defects, and prosthetic valves. Due to the increasing use of prosthetic valves, the expanding availability of dental care and the emergence of antibiotic resistant oral viridans streptococci, the incidence of this disease can be expected to increase in the United States. However, the mechanisms of disease initiation are unknown. Because (i) viridans streptococci are generally non-pathogenic in the oral cavity but virulent after entering the bloodstream, and (ii) they commonly infect valve-defected hearts but rarely infect the healthy ones, we hypothesize that certain virulence genes may not be induced until these organisms enter the bloodstream, especially if the host has a heart murmur (valve defect). Genes induced in the blood may function in concert to make the organism capable of resisting the humoral immune system, colonizing the heart valves and damaging the endocardium. Identification of these genes may ultimately lead researchers to more effective ways to prevent this disease. The aims of this proposal are to (i) construct a special integration plasmid carrying dual promoterless reporter genes, (ii) use this plasmid for selecting streptococcal genes induced in the blood of normal and valve- defected hosts, (iii) clone and begin to sequence portions of these genes. In future studies, isogenic mutants will be constructed in which specific genes are inactivated for evaluation of their roles. This is an initial step toward characterizing host blood-induced genes of viridans streptococci, and it will provide significant preliminary information for an advanced research plan (ROl). Furthermore, the proposed approach may also be useful for studies of other severe infections caused by diverse groups of streptococci and related bacteria.